A semiconductor laser module is known which includes a combination of a laser diode and an optical fiber having a grating provided therein for stabilizing the wavelength of a laser output (See document 1 as an example).
Also, a technique is known for using a wavelength converter to carry out an action of wavelength conversion from a laser beam of which the wavelength has been fixed by a grating to harmonics (See document 2 as an example)
Moreover, a technique is known for modifying the temperature of a wavelength converter to produce an output of harmonic light at stable level regardless of changes in the ambient temperature and the wavelength of a laser diode (See document 3 as an example).
Moreover, a technique is known where the output of laser beam is controlled by adjusting a current for driving a semiconductor laser with a measurement, detected by a detector, of the intensity of a portion of harmonic light released from a wavelength converter and separated by a splitter (See document 3 as an example).
Document 1                Japanese Patent No. 3120828        
Document 2                Japanese Patent No. 3223648        
Document 3                Japanese Patent Laid-open Publication (Heisei) 5-53163        
Document 4                Japanese Patent Laid-open Publication 2000-138405(Problems that the Invention is to Solve)        
It is assumed that the wavelength bandwidth of the fundamental wave of light released from an optical fiber having a grating part provided therein and received by a wavelength converter is 0.6 nm and the center wavelength λi of the released wavelength range is varied by δλi=0.01 nm/° C. depending on the temperature of the grating part.
Also, the acceptable wavelength bandwidth of the wavelength converter is 0.1 nm, and the center wavelength λc of the acceptable wavelength range is varied by δλc=0.06 nm/C.° depending on the temperature of the wavelength converter.
Accordingly, even if the center wavelength λi of the released wavelength range of the fundamental wave of light released from the optical fiber is equal to the center wavelength λc of the acceptable wavelength range of the wavelength converter when the temperature of the grating part and the temperature of the wavelength converter are at a reference temperature To, while the temperature of the grating part and the temperature of the wavelength converter are changed by ΔT in the ambient temperature change, the center wavelength λi of the released wavelength range of the fundamental wave of light released from the optical fiber is completely deviated from the center wavelength λc of the acceptable wavelength range of the wavelength converter, thus allowing no output of harmonic light.ΔT={(released wavelength range of the grating part)/2+(acceptable wavelength range of the wavelength converter)/2}/(δλc−δλi)={(0.6/2)+(0.1/2)}/(0.06−0.01)=7° C.
However, none of the prior arts is successfully designed for overcoming the foregoing problem.
On the other hand, the inventors have developed a solid laser apparatus which produces a near ultraviolet ray or visible light by emitting laser light of red color to near infrared ray by a semiconductor laser, stabilizing in the wavelength by an optical fiber having a grating part provided therein and generating second harmonic of the fundamental wave of the laser light from the optical fiber by a wavelength converter which has a structure of periodically poled non-linear optical crystal. And in view of the demand for linearly polarized waves particularly in the measuring field or bio-engineering field, the second harmonic of linearly polarized light is created by setting the crystal axis and the direction of polarization of the wavelength converter to convert the extraordinary fundamental wave into the extraordinary second harmonic. Also, the intensity of output light is controlled by separating a portion of the harmonic light emitted from the wavelength converter by a splitter, measureing the intensity of the separeted light by the detector and modifying the current for driving the semiconductor laser with a measurement result.
However, the intensity of a desired output, that is, linearly polarized light may hardly be controlled depending on the wavelength of the fundamental wave.
It is hence an object of the present invention to provide a solid laser apparatus which can produce an output of harmonic light at stable level regardless of any change in the ambient temperature and can eliminate the occurrence of a state where the intensity of a desired output of light, that is, linearly polarized light fails to be properly controlled.